The present invention is directed to the treatment of waste materials containing organic matter, such as biosolids resulting from the biological treatment of wastewater. More particularly, the present invention is directed to a process and system for treating waste materials containing organic matter by adding alkaline materials such as lime to the waste materials, and further subjecting the treated waste to energy waves such as electromagnetic waves in the microwave and/or radiowave frequency ranges to produce a xe2x80x9cClass Axe2x80x9d sludge product, as defined by 40 C.F.R. xc2xa7503 (2002), which then may be beneficially applied to land as, for example, a fertilizer or soil amendment.
The disposal of waste materials presents many concerns for the environment and the public health. Waste materials, such as wastewater treatment plant sludge, animal excrement, food wastes, and process wastes, may contain bacterial or viral pathogens and microorganisms, which, if left untreated, may pose a biological hazard to humans and the environment. Further, untreated waste materials, as well as substantially sterilized waste materials, have little or no commercial value and are typically disposed of in landfills, thereby adding to their high cost of disposal. However, in addition to unwanted pathogenic microorganisms, these waste materials often contain beneficial agricultural fertilizer nutrients such as nitrogen and phosphorus. Therefore, treatment of waste materials in a manner that produces a product satisfactory for agricultural, horticultural, or other public use, e.g. as a soil conditioner or fertilizer, may be a highly desirable form of solid waste management and may provide many beneficial uses while protecting the environment and public health. Further, as current waste treatments can take days, or even weeks, to produce a beneficial and useable treated end product, there is a need for a quick and economical waste treatment process.
Certain waste materials are subject to regulation by governmental agencies that impose restrictions on their use depending on pathogen levels and vector attraction. These agencies have established regulations for pathogen levels and vector attraction after treatment which dictate whether the resultant treated waste may be used for agricultural or other public use, or whether the treated waste must be disposed of in a landfill.
For example, the United States Environmental Protection Agency (USEPA) defines the pathogen levels and vector attraction in two different classes for regulating waste treatment methods at 40 C.F.R. xc2xa7503 (2002). A xe2x80x9cClass Axe2x80x9d sludge, according to the USEPA regulations, is a sludge that has been treated by one of six alternative processes to either reduce the density of fecal coliform in the sewage sludge to less than 1,000 most probable number (MPN) per gram total solids (dry-weight basis), or reduce the density of Salmonella sp. bacteria in the sewage sludge to less than 3 MPN per 4 grams of total solids (dry-weight basis). 40 C.F.R. xc2xa7503.32 (2002). One of these six alternatives is a xe2x80x9cProcesses to Further Reduce Pathogensxe2x80x9d (PFRP). In addition, to qualify as a Class A sludge, the sludge must be shown to have reduced attractiveness to vectors, for which USEPA regulations provide 10 options for reducing vector attraction. 40 C.F.R. xc2xa7503.33 (2002). Class A sludge, such as one treated using a PFRP, has fewer restrictions on its public use, and therefore can be used as a soil conditioner or fertilizer to fertilize cultivated fields, crops, golf courses, and public common areas such as parks.
A xe2x80x9cClass Bxe2x80x9d sludge, as defined by 40 C.F.R. xc2xa7503.32 (2002), is a sludge that may have higher levels of pathogens and consequently, more restrictions on its application to land. A Class B sludge may be achieved by one of three alternatives, one of which is a xe2x80x9cProcess to Significantly Reduce Pathogensxe2x80x9d(PSRP). Processes approved under the more lenient PSRP level have higher pathogen thresholds. As a result, public use of waste treated under PSRP may be more restrictive, making its application to land, such as for agricultural or other public use, impractical. Thus, a PSRP end product is generally disposed of in a landfill, resulting in costly landfill fees.
Several patents disclose methods for biosolids waste treatment in which the treated waste material can be subsequently utilized for agricultural purposes and to condition soil. Pasteurization, one well-known method to treat waste, involves exposing the waste material containing pathogens to an elevated temperature for a specified period of time, thereby destroying or sterilizing harmful pathogenic microorganisms. Depending on the length of time permitted for the pasteurization reaction and the amount of reactive compounds (e.g. alkaline materials such as alkaline earth metal oxides) added to the waste materials for pasteurization, the pasteurization temperature can reach over 100 degrees Celsius. However, should the temperature of the waste material exceed 80 degrees Celsius for an extended period of time during the pasteurization process, the waste material may become sterilized from the excessive heat, thereby rendering it less beneficial for agricultural purposes. As a result, it is desirable to more closely control the temperature range during pasteurization in order to pasteurize at the optimum time and temperature for effective pathogen reduction while producing a safe and useable Class A sludge end product.
However, controlling the temperature during pasteurization may be difficult as the pasteurization temperature is dependent on several factors, including, interalia, the amount of reactive compounds added, the percent by weight of liquid in the waste material, and the length of time of the pasteurization reaction. Wide variations in the oxide content of the reactive compounds and the liquid content in each batch of waste material make it difficult to gauge the proper ratio of waste materials to reactive compounds for a complete and uniformly controlled pasteurization reaction. As a result, it is often difficult to control the pasteurization temperature at the desired level. To ensure that sterilization by pasteurization at elevated temperatures does not occur, longer pasteurization processing conducted at a lower temperature have been utilized. Some pasteurization processes are conducted over days or weeks. This adds to the overall cost of waste treatment by increasing process time.
An example of an existing waste treatment method involving pasteurization is disclosed in U.S. Pat. No. 5,013,458 to Christy (the ""458 patent). The ""458 patent discloses a pasteurization method that involves retaining heat from an exothermic reaction and adding additional, supplemental heat from heating elements to neutralize the pathogens. The exothermic reaction is caused by adding an alkaline substance, such as calcium oxide, calcium carbonate, lime or quicklime, to the waste materials in sufficient quantity to raise the pH level of the waste.
Pasteurization is achieved by allowing the waste materials and alkaline substance to exothermically react in a substantially closed container for at least two hours, generating and releasing heat. The patent also discloses that the longer the reaction is held, the greater the degree of lime stabilization, and that the reaction time can be held for a number of days, or even weeks. By maintaining the temperature of the waste mixture at no less than 70 degrees Celsius for at least one half hour, the waste mixture is pasteurized to Class A sludge. External supplemental heating is supplied by placing heating elements evenly spaced around the exterior of a vessel. These intermittently-spaced supplemental heating elements can cause uneven heating, which can create sterilizing hot spots and/or underheat other areas resulting in ineffective pathogen kill.
Another example of an existing waste treatment method involving pasteurization is shown in U.S. Pat. No. 5,196,043 to Wurtz (the ""043 patent). This patent describes a pasteurization waste treatment process that involves reacting the waste materials with alkaline earth metals and carbon dioxide, thereby producing two exothermic reactions which result in a useable Class A sludge product. But the ""043 patent discloses a delayed exothermic reaction that requires a reaction time ranging up to 20 minutes in which the temperature can exceed 100 degrees Celsius. Thus, neither of the above referenced patents teach a quick, economic, tight temperature-controlled pasteurization method in which the waste materials are pasteurized at a lower temperature to produce a Class A sludge.
Accordingly, there is a need for an expeditious and economical method to treat waste materials to produce products beneficial for agricultural or other public use.